The present invention relates to power machinery. More particularly, the present invention relates to an apparatus for controlling movement of a boom on a power machine.
Power machines, such as skid steer loaders, typically have a frame which supports a cab and a moveable lift arm which, in turn, supports a work tool such as a bucket. The moveable lift arm is pivotably coupled to the frame of the skid steer loader by power actuators which are commonly hydraulic cylinders. In addition, the tool is coupled to the lift arm by another power actuator which is also commonly a hydraulic cylinder. An operator manipulating the skid steer loader raises and lowers the lift arm, and manipulates the tool, by actuating the hydraulic cylinders coupled to the lift arm, and the hydraulic cylinder coupled to the tool. When the operator causes the hydraulic cylinders coupled to the lift arm to increase in length, the lift arm moves generally vertically upward. Conversely, when the operator causes the hydraulic cylinders coupled to the lift arm to decrease in length, the lift arm moves generally vertically downward. Similarly, the operator can manipulate the tool (e.g. tilt the bucket) by controlling the hydraulic cylinder coupled to the lift arm and the working tool to increase or decrease in length, as desired.
It is desirable that, under certain conditions, the lift arm or the tool or both be rendered inoperable. For example, in some prior devices, when an operator leaves the cab of the skid steer loader, the hydraulic cylinders used to raise and lower the lift arm are locked out of operation. In such prior devices, an operator presence switch is coupled to the hydraulic circuit controlling the hydraulic cylinders to render the hydraulic lift cylinders inoperable when the operator presence switch indicates that the operator is not present. One example of such a system is set out in the Minor et al. U.S. Pat. No. 4,389,154.